Generally, during a high speed search in a digital audio tape recorder, the running velocity of the tape is increased up to 200 times that of the normal playing velocity. But the head drum revolution velocity is not increased to 200 times in such a case, and therefore, the head drum revolution velocity has to be increased when the regeneration clock frequency becomes same as the normal regeneration frequency. It takes much time to increase the revolution velocity of the head drum, and therefore, the start-up time is extended, thereby making it impossible to carry out dexterous high speed searches.
FIG. 1 is a block diagram of the conventional head drum servo control device in which, during a high speed search, a reference frequency f1 is supplied to a frequency comparator 1, and a radio frequency (to be called hereinafter "RF") generated during the revolution of a head drum 6 is supplied to an RF amplifier 5. The RF amplifier 5 amplifies the RF signals, and these RF signals are supplied to the frequency comparator 1 through a phase locked loop 2 (to be called hereinafter PLL) for tracing the regeneration clock frequency. The frequency comparator 1 compares the incoming two signals, supplies output error signals to an adder 3, and supplies also to the adder 3 the output signals VH of a ramp voltage generator 7 for generating a ramp voltage during a search. The two kinds of signals inputted into the adder 3 are added together, and the added signals are amplified at a certain amplifying rate by an amplifier 4 in order to be outputted to the head drum 6. The head drum 6 is controlled to a revolution velocity equivalent to the driving voltage signal inputted therein. The ramp voltage generator 7 stops the voltage generation upon the receipt of control signals VC from the PLL 2 after being switched over to the normal running mode, and maintains the output voltage of the stopping moment.
As shown in FIG. 2 where the operating states of the reel and drum during a high speed search are illustrated, if the operation of a high speed search is started at a point a, then the point a shows the states of the drum control voltage VD and the reel control voltage VT corresponding to the variations of their revolution velocities. A point b is the point where the target level of the running velocity of the tape is attained, that is, the acceleration of the tape velocity is completed at the point b. A section c represents the interval where the revolution velocity of the head drum is subjected to a sweeping-up in order to control the relative velocity of the head drum at a constant level in correspondence with the acceleration of the reel velocity. A section f represents the interval where the revolution velocity of the head drum is subjected to a fine adjustment, while a point d represents the position where the revolution velocity of the drum is set.
That is, in the intervals c and f, controls are made in such a manner that the regeneration clock frequency of the regeneration data becomes same as the regeneration clock frequency of the normal running mode. The velocity of the head drum has to be increased until the two frequencies become equal each other, but the velocity of the head drum is increased in a state in which the head drum control voltage outputted from the ramp voltage generator 7 does not have any particular target level, and therefore, the interval c of FIG. 2 is lenghtily extended, thereby making the high speed search operation slow.
A servo control device for use in a digital audio tape recorder DAT is disclosed in Japanese Laying-opening No. Sho-63-153759. In the above invention, in order to overcome the image collapse in the transition from the normal regeneration to a high speed search, the revolution velocity of the head drum is increased step by step, and, in accordance with the variation of the revolution velocity of the head drum, the running velocity of the tape is servocontrolled by a constant relative velocity of the head drum, with the result that image collapse is prevented, and that the transition to a high speed search can be carried out.
However, while the image collapse can be prevented in the above described method, the start-up time becomes extended, and therefore, truly high speed searches are not realized.